1. Field of the Invention
The present invention relates to disk drives and, in particular, to a demarcation circuit that identifies when the last byte of data prior to a servo block has been read, regardless of the location of the servo block.
2. Description of the Related Art
Magnetic recording disks store data in a series of concentric recording tracks which are each logically divided into a number of sectors. Each recording track is also divided by a number of servo blocks which are used by the servo head of a disk drive to control the positioning of the read/write head over the tracks.
An embedded-servo recording format is a well known disk format that positions the servo blocks so that the servo blocks within each track radially correspond with the servo blocks of the adjoining tracks. As a result, the servo blocks form a number of pie-shaped regions that extend radially outward from the center of the disk. With the embedded-servo format, the sectors are typically positioned to follow immediately after a servo block, thereby allowing the read/write head of the disk drive uninterrupted access to the sector.
A zoned-data recording format is another well known disk format that groups a number of adjoining tracks together to form zones. By forming zones, a large number of equally-sized sectors can be formed on a disk. The zoned-data recording format takes advantage of the fact that the circumference of the tracks, and thereby the recording area, increases as the tracks move radially outward. Thus, for example, an inner zone may have 50 n-bit sectors per track while an intermediate zone may have 75 n-bit sectors per track and an outer zone 100 n-bit sectors per track.
One problem with using zoned-data recording on disks that also employ embedded servo blocks is that the servo blocks typically end up "splitting" the sectors in an irregular fashion in all but the most inner zones. FIG. I shows a timing diagram that illustrates the relationship between sectors and servo blocks when an embedded-servo, zoned-data recording format is utilized.
As shown in FIG. 1, a first servo block 10 occurs approximately in the middle of a sector n while a second servo block 12 occurs just after the beginning of a sector n+1. Thus, when the read/write head of a disk drive is reading from a "split" sector, the data path control circuitry of the disk drive must be able to determine where the data ends and the servo block begins to avoid reading in unwanted data.
The most common solution to this problem is to determine the number of bytes that are recorded between the beginning of each sector and the beginning of the servo block that splits the sector, and between the end of the servo block and the end of the sector, and then store these values as a byte number in the ID field of the sector. In operation, a demarcation circuit reads in the byte number and then counts the bytes of data as they are being read from the disk. Thus, when the count matches the byte number, the last byte of data prior to the servo block has been read and the demarcation circuit can suspend operation of the read/write circuitry.
The principle disadvantage of this solution is the lost recording area that results when the position of a servo block causes the ID field to be "split". If an ID field were allowed to be split, the demarcation circuit could not read the byte number, and therefore could not determine the number of bytes to count.
Thus, as shown in FIG. 1, when a servo block splits an ID field, the beginning of the sector is typically adjusted so that the sector begins after the end of the servo block. As a result, the recording area between the original beginning position of the sector and the adjusted beginning position is typically lost. In addition, recording area is also lost as a result of the extra overhead that is required to store the byte number.
Another solution to this problem is to utilize a demarcation circuit that counts the number of reference clock signals that occur between successive servo blocks. Since the number of reference clock signals which can occur between the servo blocks of a specific track at a specific clock frequency is constant, there is no need to store a count number in the ID field. Further, since the ID field is no longer being utilized to store a count number, a servo block which splits the ID field no longer presents a problem.
Thus, with this approach, a demarcation circuit reads an internally-stored clock count number and then counts the reference clock signals as they are detected. When the count matches the clock count number, the last byte of data prior to the servo block has been read and the demarcation circuit can suspend operation of the read/write circuitry.
The principal disadvantage of this approach is that the reference clock signal, which is an externally-generated, crystal-controlled clock signal, does not accurately represent the timing of the data as it is being read from the disk. This lack of accuracy results from slight variations which occur in the spindle speed of the typical disk data recovery device.
As is well known, data is written to a disk by writing one bit of data for each n reference clock periods, i.e., one bit of data per each clock period. When data is then written to a disk at a constant spindle speed, each bit of data consumes a specific amount of disk space. As the spindle speed varies, however, more or less disk space is consumed. As a result, more or less time is required to read back each bit of data.
Thus, although the time required to count x reference clock signals will remain constant, a timing error will result because the number of bits of data which will have been read during that time will vary as a function of the variation of the spindle speed. As a result, the reference clock signal cannot be utilized to accurately identify when the last byte of data prior to a servo block has been read. Thus, there is a need for a demarcation circuit that can accurately read the data in a sector regardless of which field is split by a servo block.